1. Field of Invention
This invention relates to medical apparatus for use in the implantation of a joint prosthesis to the end of a bone and, more particularly, to a plug for stopping the flow of bone cement, used in the attachment of a prosthesis, at a predetermined point within the intramedullary bone channel.
2. Description of the Related Art
A number of U.S. Patents describe apparatus for plugging the open end of an intramedullary bone canal to restrict the flow of bone cement during the insertion of bone cement, particularly during the pressurized injection of such a cement during an operative procedure involving the fixation of the stem of an artificial joint prosthesis to the end of a bone such as in the fixation of a hip joint prosthesis to the proximal femur.
For example, U.S. Pat. No. 4,697,584 describes an inflatable bone plug which is inflated with a fluid capable of escaping from the plug within a relatively short period of time after the bone cement holding the prosthesis in place has hardened to avoid possible weakening of the cortical bone surrounding the inflated plug. This invention also relates to an inflatable bone plug of the above type which is preferably constructed from a silicone elastomer and is pressurized with carbon dioxide gas. However, for substantial inflation to occur in the desired manner, the bone plug is made of an elastomeric material. Such materials, which have elastic properties allowing substantial elongation, are subject to cutting and puncturing due to bone splinters and sharp edges in a channel cut into bone, as the plug is moved into position within the channel. Furthermore, the use of an internal valve associated with a needle for injecting a fluid complicates the manufacture of the inflatable bone plug.
U.S. Pat. No. 5,849,014 describes a cement restrictor system including an inflatable body, a conduit having first and second ends that defines a fluid passage to and from the inflatable body, and a shield releasably securable to the conduit. In an exemplary method of making a cement plug with the system an obstruction, such as the shield, is placed in a medullary canal of a long bone beyond the isthmus of the long bone. The obstruction is held in place with the inflatable body. A predetermined quantity of bone cement is poured into the medullary canal and localized by the obstruction. The bone cement is allowed to harden; and the conduit and inflatable body are removed from the bone. Again, the inflatable body is elastomeric and is not protected from cutting or puncturing by bone splinters and sharp edges as the restrictor system is moved into place within a bone channel, with the inflatable body preceding the shield. Also, the separate formation of a bone plug from cement and subsequent removal of the conduit and inflatable body to allow insertion of the prosthesis may lengthen the time required for a hip replacement procedure.
U.S. Pat. No. 5,935,169 describes a bone cement plug including a core having a substantially cylindrically-shaped base portion defining a threaded bore therein extending axially and distally from a proximal end of the base portion; a first leg portion depending from and extending distally from the base portion; and a second leg portion depending from and extending distally from the base portion and opposed to the first leg portion; the base portion threaded bore being adapted to receive an expander screw to wedge apart the first and second leg portions, whereby to expand the core widthwise to secure the plug in the bone canal; and the expander screw, the screw comprising a generally cylindrically-shaped body having a tapered distal end, and a proximal end in which is disposed a threaded bore, external threads disposed on the body, and an annular flange extending outwardly from the proximal end of the body, the screw being threadedly engageable with the core threaded bore for advancement of the screw into the plug for the wedging apart of the first and second legs. A method is also disclosed for using the bone cement plug to compact bone cement into a bone canal during total joint replacement surgeries. However, this type of bone cement plug, having a finite number, such as two, mechanically coupled legs to expand, cannot have the flexibility of a system using an inflatable plug in terms of forming a proper seal within a bone channel which may not be round and smooth, and which varies locally in radius.
U.S. Pat. No. 5,997,580 describes a cement restrictor including a member or body that is expandable or transitionable from a first diameter to a second diameter. The cement restrictor includes a single or multiple finned body having a first stable state and a second stable state. In the first stable state, the cement restrictor is narrower than in the second stable state. While the cement restrictor is readily transitionable from the first stable state to the second stable state, the transition can be irreversible. An illustrative embodiment of the cement restrictor includes a body having a first end and a second end. Bistable fins extend radially from the body and are irreversibly movable from a first stable state to a second stable state. The fins are concave with respect to the first end of the body in the first stable state and convex with respect to the first end of the body in the second stable state. The diameter of each fin is larger in the second stable state than in the first stable state. Other embodiments of inventive cement restrictors are shown that include shape memory material that changes shape or dimension(s) in response to temperature and/or stress. However, the expansion of the cement restrictor is limited to the transition between the first stable state and the second stable state, together with elastic and plastic deformation of the material. This method thus does not offer the kind of flexibility of a system with an inflatable body in expansion to meet varying conditions within the bone channel. Furthermore, the time required to apply liquids at different temperatures to make the transitionable body perform as desired may increase the time required for hip replacement surgery.
A number of patents describe bone plugs including a central core from which a number of disks extend as fins at spaced locations. Examples of such devices are found in U.S. Pat. Nos. 5,383,932, 5,662,657, 5,766,178, 5,782,917, 5,861,043, 5,879,403. Such systems allow only deformation of the individual disks to compensate for changes in the shape of the bone channel, such as out-of-round conditions and changes in hole diameter. Therefore, such systems cannot compensate for such conditions to the extent possible with systems including inflatable bodies.